Relatively small boron additions to conventional titanium alloys provide important improvements in strength, stiffness and microstructural stability. Because boron is essentially insoluble in titanium at all temperatures of interest, the titanium boride is formed for even very small boron additions. The density of titanium boride is nearly equal to those of conventional Ti alloys, but its stiffness is over four times higher than conventional titanium alloys. Thus, titanium boride offers significant improvements in stiffness, tensile strength, creep, and fatigue properties. Since titanium boride is in thermodynamic equilibrium with titanium alloys, there are no interfacial reactions to degrade properties at elevated temperature. Further, because the coefficient of thermal expansion of titanium boride is nearly equal to values for titanium alloys, residual stresses are nearly eliminated” Taken from JOM Article May 2004 “Powder Metallurgy Ti-6Al-4V Alloys: Processing, Microstructure, and Properties”, the entire disclosure of which is incorporated by reference.
Currently two approaches appear to be used to accomplish boron addition; 1) Blended elemental addition of TiB2 and solid state reaction to produce the titanium boride which usually forms as whiskers with a 10 to 1 aspect ratio and 2) Pre-alloyed powders from a melt process.
Negatives of the blended elemental approach are the added effort to blend the powders to obtain a uniform distribution (which is never perfect) and the added time and temperature it takes the solid state reaction to transform TiB2 to TiB (1300 C for 6 hours). Also, this approach has the potential to form larger Titanium boride particles or have residual titanium boride particles that adversely affect properties. The titanium boride whiskers that are formed can lead to anisotropic properties of the part depending on the type of process used to make the part.
A negative of the pre-alloyed approach is that it has a tendency to leave large primary borides in the pre-alloyed materials that cause low fracture toughness.
Representative examples of patents related to producing metal alloys with titanium boride are the Davies et al. U.S. Pat. No. 6,099,664 issued to Davies et al. Aug. 8, 2000, in which titanium boride particles in the 1-10 micron size range are produced in a molten reaction zone. The Blenkinsop et al. U.S. Pat. No. 6,488,073 issued Dec. 3, 2002 teaches the addition of an alloy in which tantalum boride or tungsten boride particles are added to a molten alloy material to form a molten mixture which upon cooling has the boride distributed therein. Another method of making boride containing titanium alloys is disclosed in the Abkowitz U.S. Pat. No. 5,897,830 in which titanium boride powders are mixed with the powders of various constituents to form a consumable billet which is thereafter cast or melted to form the article of manufacture. Each of these processes as described in the above-mentioned patents has a variety of shortcomings, not the least of which is the imperfect distribution of the boride as well as the size of the boride particles.